An optical signal to noise ratio (OSNR) needs to be increased to increase an optical transmission distance and transmission speed. Therefore, an optical fiber is required to have low loss characteristics.
A silica core optical fiber, whose core region is substantially formed of only pure silica glass, has no light scattering due to fluctuation of added GeO2 concentration compared to a germa-core optical fiber (an optical fiber doped with GeO2 in the core). Therefore, it is known to be an optical fiber with low loss.
A demand for even lower loss is still high for both silica core optical fibers and germa-core optical fibers.
Recently, technologies of manufacturing optical fibers have become highly sophisticated, and absorption loss due to impurities such as metal oxides (MOx) or hydroxyl groups (OH) in optical fibers has almost been reduced to the limit. Most of the remaining loss is due to scattering loss accompanied by fluctuation of a structure or composition of the glass. Since the optical fiber is formed of glass, such a loss is inevitable.
Generally, it is well known in the glass industry that the fluctuation can be reduced by cooling a melt molded glass gently. Also in methods of manufacturing an optical fiber, for the optical fiber immediately after an optical fiber preform is melted and drawn by a heating furnace, methods such as adjusting the temperature using a separate furnace (a slow-cooling furnace) or slowly lowering the temperature by adding an insulating structure to the heating furnace have been studied. In such methods, an effect has been obtained to a certain degree (see Patent Documents 1 to 8 and Non-Patent Documents 1 to 5).
In addition, a method of efficiently performing the slow cooling of an optical fiber passing through a slow-cooling furnace by selecting a low thermal conductivity gas as the atmosphere inside the slow-cooling furnace has also been studied (see Patent Documents 9 and 10).